A nitride semiconductor represented by gallium nitride (GaN) has a wide band gap and is capable of emitting blue light, and thus, is widely used for a light-emitting diode (LED), a semiconductor laser (LD), and the like. For example, a white LED in which a blue LED containing GaN and a substance emitting yellow light are combined is widespread as a backlight of a liquid crystal display (LCD) of a mobile telephone and the like. Further, a white LED has the advantage of low power consumption, a long life, and the like, and thus, is expected as a light source excellent in environmental friendliness to replace a fluorescent lamp and an incandescent lamp and is vigorously researched and developed.
A crystalline film of a nitride semiconductor is formed by epitaxial growth on a surface of a substrate for epitaxial growth (hereinafter referred to as “substrate for growth”) represented by a sapphire substrate. However, there is a difference in lattice constant between the substrate for growth and the crystalline film formed on the surface of the substrate for growth, and thus, the crystalline film is formed on the surface of the substrate for growth under a state in which stress is always applied thereon.
This stress causes a large extent of warpage of the substrate for growth and the crystalline film. Due to this warpage, when an electrode pattern is formed on the surface of the crystalline film by a photolithography step in producing the elements such as LEDs, the focal length of light differs in exposure between a center portion and a peripheral portion of the substrate for growth. Therefore, it is difficult to uniformly perform the photolithography step on the surface of the crystalline film, and, as the substrate becomes as large as one having a diameter of the substrate for growth of 2 inches or larger, unevenness (nonuniformity) of the photolithography step becomes more conspicuous.
In order to prevent the unevenness, it has been devised that heat treatment of the substrate for growth and the crystalline film is carried out at a temperature that is lower than the temperature at which the warpage is completely relieved to make a substrate for growth having warpage in a direction opposite to that of the warpage caused by the epitaxial growth (see, for example, Patent Literature 1).
Further, when a crystalline film having such a thickness that the crystalline film can stand on its own is formed on a surface of a substrate for growth by epitaxial growth, due to stress caused in the crystalline film, a crack develops in the crystalline film. As a method for suppressing the crack, a method of producing a crystalline film is invented in which a mask material in the shape of a lattice is formed on the surface of the substrate for growth to form an epitaxial formation region of the crystalline film as small regions which are independent of one another (see, for example, Patent Literature 2).
Patent Literature 2 discloses a method of producing a nitride semiconductor film capable of simultaneously suppressing a crack and threading dislocation. As illustrated in FIG. 14, Patent Literature 2 discloses that a silicon oxide mask material 100 on which a nitride semiconductor film is not epitaxially grown is formed in the shape of a lattice, and, by exposing portions of a surface of a substrate 101 for growth so as to be separate from one another, the epitaxial growth region is formed as small regions which are independent of one another. By forming a reaction preventing layer 102 mainly formed of a monocrystal on the exposed portions of the surface, chemical reaction between the substrate 101 for growth and a nitride semiconductor film 103 thereabove due to stress and heat is prevented from occurring during the production process.
By forming, after this, a distortion relaxing layer 104 by alternately forming nitride semiconductor films of the same or different compositions in two different temperature ranges, stress between the substrate 101 for growth and the nitride semiconductor film 103 may be relaxed, and thus, development of a crack in the nitride semiconductor film 103 may be suppressed.